The invention relates to an arrangement for avoiding annoying, sustained oscillations in a closed-loop system, the arrangement comprising a variable-gain amplifier incorporated in a first path which forms part of the loop of the said system.
Among the closed-loop systems in which annoying sustained oscillations may be produced there are, for example, systems for controlling electrical or mechanical or physical quantities. It is known that during the design of such systems, the gain and phase conditions in the control loop must have proper attention paid to them in order to avoid sustained oscillations in the loop. But there is never a certainty that due to unforeseen circumstances, for example parasitic couplings or sudden perturbations, self-oscillations will not be produced which may attain high and even dangerous amplitudes in certain systems. A different type of closed-loop system is an electro-acoustical system which comprises, for example, a microphone and a loudspeaker which are coupled in any arbitrary manner by means of an electric circuit; if on the other hand these two transducers are coupled by means of an acoustic circuit, an electro-acoustic loop is obtained in which oscillations of a high amplitude may be produced which cause unacceptable howls. This phenomenon, which is known as the Larsen effect, may be produced in, for example, sound reproduction equipment or in loudspeaking telephone sets.
To avoid oscillations in an electro-acoustical loop the procedures which are generally employed include the provision in the loop of at least a variable amplifier or attenuator circuit, which is controlled in accordance with different criteria in order that the gain in the loop remains below unity. Thus, a procedure employed in loudspeaking telephone sets consists of checking, by means of two envelope detectors, whether the speech signal is present in the loudspeaker path or in the microphone path and producing a gain increase in the useful path and a gain reduction in the other path. These gain modifications which follow each other in both paths during a conversation are very unpleasant for the listeners and in addition this type of procedure is not very effective for the important acoustical couplings, as the gain control does not substantially depend on the coefficient of acoustic coupling.
Another procedure described in French Patent Application No. 2,461,412 also employs two signal envelope detectors in the two paths and a single amplifier in the loudspeaker path, the gain of which is varied by a difference signal between the output signal of the envelope detector of the microphone path and the output signal of the envelope detector of the loudspeaker path, weighted with a predetermined coefficient. For distances which are longer than a predetermined minimum distance between the loudspeaker and the microphone, the gain of the amplifier is in a constant ratio to the coefficient of acoustic coupling and the oscillations can only be avoided by means of this procedure below said minimum distance. In addition, the use of envelope detectors causes the reception to depend on the speech signals and particularly on the speech signal produced by a speaker who speaks into the microphone.